It is known that a product having the uneven microstructure regularly disposed nano-sized fine roughness on the surface is continuously shifting the refractive index and then exhibits anti-reflect performance. It is also known that the product having the uneven microstructure exhibits the same effect as super hydrophobicity (lotus effect) which lotus leaves having fine downy hairs are showing.
As a method for producing articles having uneven microstructure on the surface, the following methods have been proposed:
(i) a method of using a stamper having the inverse structure of a uneven microstructure on the surface and transferring the uneven microstructure onto the surface of thermoplastic resin moldings by injecting or pressing a thermoplastic resin;
(ii) a method of transferring the uneven microstructure onto the cured product which comprises filling an active energy ray-curable resin composition between a stamper having the inverse structure of uneven microstructure on the surface and a transparent base material, curing it by irradiation with an active energy ray, and then removing the stamper; and(iii) a method of filling an active energy ray-curable resin composition between the above stamper and a transparent base material, then removing the stamper to transfer the uneven microstructure onto the active energy ray-curable resin composition, and subsequently, curing the active energy ray-curable resin composition by irradiation with an active energy ray.
Among them, the method (ii) curing the resin composition by irradiation with the active energy ray to transfer the uneven microstructure is suitable, in consideration of transferability of the uneven microstructure and optionality of the surface composition. This method is particularly suitable for use of a belt- or roll-type stamper enabling continuous production, and is also a highly productive method.
The uneven microstructure exhibits high anti-reflective performance, when the interval between adjacent convex or concave portions of the micro-relief is equal to or less than wavelength of visible light. However, the product having the uneven microstructure having such a structure is in scratch resistance inferior to the articles which have smooth surface and are treated for abrasion resistance with a hard coat or the like, and has problems with durability in use. Moreover, if the resin composition used for production of the product having the uneven microstructure has not well solid, a phenomenon of projections gathering together occurs in removing from the mold or by heating.
Then, to maintain the uneven microstructure, a product having the uneven microstructure formed by curing a resin composition by irradiation with an active energy ray and transferring the inverse uneven microstructure of a stamper, and resin compositions for forming such an uneven microstructure were proposed.
For example, it is known to produce a product having the uneven microstructure having an interval between convex portions (concave portions) of not more than the wavelength of visible light using close-packed silica sol as a mold, and to use a multifunctional monomer inclosing extremely large number of double bonds per molecular weight such as trimethylolpropane triacrylate in the resin composition which is used for forming the uneven microstructure (Patent Literature 1).
Further, a film having a hard coat layer having an uneven microstructure is disclosed. The hard coat layer is formed of a resin exhibiting a hardness of “H” or more in a pencil hardness test according to JIS K5600-5-4. And the resin is produced using a multifunctional monomer having an extremely large number of double bonds per molecular weight such as dipentaerythritol hexaacrylate, dipentaerythritol pentaacrylate and pentaerythritol tetraacrylate (Patent Literature 2).
Meanwhile, as preferable resin compositions for forming the uneven microstructure followings are known:
(1) a photo-curable resin composition containing an acrylate oligomer such as urethane acrylate and a release agent as essential components (Patent Literature 3);
(2) a photo-curable resin composition composed of a (meth)acrylate such as ethoxylated bisphenol-A di(meth)acrylate, a reactive diluent such as N-vinylpyrrolidone, a photoinitiator, and a fluorosurfactant (Patent Literature 4); and
(3) an ultraviolet light curable resin composition comprising a multifunctional (meth)acrylate such as trimethylolpropane tri(meth)acrylate, a photoinitiator, and a leveling agent such as polyether-modified silicone oil (Patent Literature 1).
However, the products having the uneven microstructure described in Patent Literatures 1 and 2 each are a cured product having a high crosslinking density and a high elastic modulus, but do not always satisfy scratch resistance. Even if a cured resin is a hardness of “H” or more in the pencil hardness test, the product having the uneven microstructure particularly is impaired the anti-reflective performance by breakage or bending of fine projections of the product having the uneven microstructure, and therefore, its uses are limited. It is also difficult to wipe off dirt adhered on the uneven microstructure such as fingerprints.
On the other hand, for antifouling property of resin molded articles, a method for suppressing adhesion of dirt by providing hydrophobicity thereon using a silicone- or fluoro-compound. On the contrary, a method for making the surface of the resin molding hydrophilic to promote soaking water between the adhered dirt such as fingerprints and the surface of the resin, and floating the dirt with water by water wiping also presents. However, making the surface of a resin molded article hydrophilic naturally results in enhancement of moisture absorbing properties of the resin molded article and softening of the molded article, and the uneven microstructure formed on the surface of the molding trends to be impossible to be maintained.
Since the cured products described in above Patent Literatures 1, 3 and 4 are insufficient hydrophobic, even if trying to remove the dirt adhered on the surface of the uneven microstructure such as fingerprints by water wiping the dirt, the dirt does not float up with water and fingerprints and the like are difficult to be wiped off. On the other hand, the simply increased hydrophobicity of the resin only increases absorption of water by the resin, and a softened molded article will be obtained. If a molded article having a nano-order uneven microstructure on the surface is formed using such a resin, the nano-order projections gather together caused by surface tension and scatter light to reduce the reflection suppressing effect and have problem with falling white and hazy after water-wiping.